Solid state power controllers (SSPCs) are used as an alternative to mechanical relays and circuit breakers to distribute power and protect loads. SSPCs have found use, for example, in the power distribution within aircraft. An SSPC typically makes use of one or more solid state switching devices to provide on and off control of the power delivered from a power bus to a load. The solid state switching devices used in SSPCs are typically field effect transistors (FETs), and more particularly metal oxide semiconductor field effect transistors (MOSFETs).
There has been a trend toward using SSPCs with increasingly large loads. The increased current carrying capacity needed to control larger loads has been addressed by current sharing using multiple current paths, with each path containing a MOSFET to control flow of the current. The number of parallel current paths used by the SSPC may vary from a few to many.
MOSFET based SSPCs normally operate the MOSFET in the fully-ON saturated state where only the drain-source ON resistance (Rds-ON) affects the voltage drop and power dissipation in the MOSFET. In this state, multiple MOSFETs in parallel current paths can share a load effectively; and since the MOSFETs are saturated when ON, the total power dissipation in the ON state is relatively low.
However, during turn-off of an inductive load, the MOSFETs have to spend some time in the linear operating region with both substantial current flowing through the MOSFETs and at a substantial voltage drop between drain and source until the energy stored in the inductive load has been dissipated. During the turn-off period, if one of the MOSFETs carries a substantially larger share of the total current than other MOSFETs in the other parallel current paths the MOSFET carrying a larger share of the current could be exposed to very high peak power dissipation levels and as a result could be damaged. The most common cause of this imbalance in sharing is differences between the devices in the Gate threshold voltage.